BACTERIA:

INTRODUCTION: Bacteria are the prokaryotic micro-organisms, most of which do not contain any membrane bound nucleus or other membrane bound cell organelles. According to the five Kingdom classification, The prokaryotes were placed within the kingdom Monera and protista. But later on, started the prokaryotic controversy, according to which, there are certain bacterial phyla such as the planctomycetes, which had their genetic material enclosed within a membrane and also other membrane bound cell organelles such as the anammoxosome(the site for oxidation of ammonia) and this was not in accordance with the prokaryotic definition. Hence, two new domains namely Bacteria and Archaea came into existence into which these prokaryotes were placed. These bacterial cells are in different shapes and arrangements  which are explained as follows:

Shapes and Arrangement of Bacteria cells: Bacteria exist in a variety of shapes and arrangement of their cells. The surface area to the volume ratio of these cells played an important role in determining their shape. As a part of the evolutionary process, Bacteria started attaining different shapes which favored more  uptake of nutrients and in turn their growth.The planes of their division decided their arrangement of the cells. Given below are the types of shapes and arrangements of the bacterial cells with examples;

Cocci: The Bacteria first existed in round shape called the Cocci and these cocci in pairs are called diplococci. When the cocci adhere to each other after repeated divisions, they form chains of cells(Examples: Streptococci, Enterococci) When the division takes place randomly then, these cocci cells exist in the form of bunches like grapes(Staphylococci is an example of such bacteria).  Apart from these types, there are also tetrads of cocci formed by the division of these cocci in two planes forming square groups of four cells(Example: Micrococcus), and also cubical packets of eight cells formed by the division in three different planes(example: Sarcina).

Bacilli: These are rod shaped bacterial cells which differ from the cocci in the length-to-width ratio and they can take up more nutrients and grow well because of their shape and size. These rods may exist either in flat, rounded or cigar-shaped. They also sometimes form pairs or chains similar to the cocci. Lactobacillus which plays an important role in the formation of curd is a well known example of Bacillus and Bacillus megaterium is found in long chains.

Vibrios, Spirilla and Spirochetes: Apart from the rods and cocci, There are also some other shapes of bacteria which includes the Vibrios, Spirilla and Spirochetes. The Vibrios are the comma shaped ones which include the Cholera Vibrio, The causative agent of cholera(a water borne bacterial infection). Spirilla and Spirochetes are spiral shaped cells with a difference in their flexibility. Spirilla are rigid and have flagella either at one or both of its ends. Flagella are the locomotary organs which help in the movement of these bacteria. Spirochetes are flexible spiral shaped cells which have internal flagellar arrangement. Some bacteria are also Pleomorphic which can exist in various shapes and do not have a single form.

Filamentous bacteria: There are also some bacteria which form long filaments, the filaments being called as hyphae. Cyanobacteria are an example of filamentous bacteria that participate in the process of photosynthesis. The hyphae branch out to form structures called mycelium and due to this reason, they are similar to filamentous fungi.

Size of bacteria: There is a wide range of the size of bacterial cells, with the smallest one belonging to the genus Mycoplasma(0.3 micrometres) and the huge one being Epulopiscium fishelsoni (about 600 by 80 micrometres). Thiomargarita namibiensis is the larger bacterium found in the sediments of the ocean which is thought to have a size greater than a eukaryotic cell. With larger size of bacteria, there is more nutrient influx and more growth of these cells. E-coli is the average sized bacterium with the length between 2.0 and 6.0 and width between 1.1 and 1.5 micrometres.

Composition of bacterial cells:  The cells of bacteria are simple in their structure and they have their genetic material interwound inside a structure called the nucleoid. If we consider the cytoplasm as the main body of the cell, then the nucleoid is the brain of them. Their cells are surrounded by a typical cell wall which provides protection against the external forces and has many structures such as the peptidoglycan layer, the lipopolysaccharides embedded within it that account for its toxicity. Underlying the cell wall is the plasma membrane which is a semi porous membrane that provides a particular shape to the cell and  is also responsible for the transport of various substances like gases, nutrients etc into the cells. There are some structures such as the ribosomes, large masses called inclusion bodies etc. which are also scattered in the cytoplasm of the cell. Ribosomes are the main sites of protein synthesis. There are also structures like capsules or slime layers surrounding the bacterial cells, which help in the protection against phagocytosis and adhesion to surfaces. Some bacteria have internal structures called endospores which are very strong and can survive in harsh environments and cannot be destroyed easily as a result of which, they can cause many infections. Flagella, fimbriae, cilia are the locomotary organs which help the bacteria move from one place to the other and also to adhere to the surfaces.

References:

Joanne M. Willey, Linda M. Sherwood, Christopher J. Woolverton. Prescott's Microbiology Ninth Edition. United states of America: McGraw-Hill, 2014.Print.